In a plurality of the wireless communication systems presently used, such as GSM, frequency hopping is utilized to achieve frequency diversity and interference diversity. Frequency diversity is wanted primarily to mitigate the effects of Rayleigh fading, i.e. that for given positions of a transmitter and a receiver a signal of certain frequency may experience a substantial attenuation, or fading, while a signal of another frequency is substantially less attenuated. Interference diversity, in particular inter-cell interference diversity can be used to increase the capacity of, and/or optimize the transmission power in, the communication system.
The principles of frequency hopping as used in the prior art will be exemplified primarily with GSM. Prior Art FDMA systems utilizes a fixed bandwidth for each carrier, in GSM corresponding to 200 kHz, which is shared between 8 possible users in a time division multiple access (TDMA) arrangement. Arguable a frequency range is used for each communication between a base station and a mobile station; however, the term commonly used is that a frequency f1, also referred to as carrier or carrier frequency, is used for the communication, and another frequency f2 is used for the communication between the base station and another mobile station, for example. Each cell/base station has a number of carriers, or frequencies, assigned to it, in a pattern chosen to minimize interference in-between cells. Upon setting up a call, the mobile station is assigned a frequency: the frequency f1, and the timeslot T3, for example. And an identifier of the frequency hopping sequence used for that connection, and the time of its frequency hopping clocking in order to enable that the mobile station and the base station use the same frequency at the same time. The carrier frequencies are changed in predetermined patterns according to the identified frequency hopping sequence. In GSM 64 different patterns are provided for and the selection has to be communicated to all entities affected by the frequency hopping.
The above described possible advantages with frequency hopping is recognized in the art and frequency hopping is provided for in a plurality of presently used, or proposed, systems. As the bandwidth is fixed, the implementation is rather straightforward: the system changes carrier frequencies among the frequencies assigned to the cell during a communication. A comprehensive overview of frequency hopping and its advantages in GSM is to be found in “Interference Diversity as Means for Increased Capacity in GSM” by H. Olofsson et al., Proceedings of EPMCC'95.
The frequency hopping methods outlined above may be extended also to certain multi-carrier systems with variable transmission bandwidth, i.e. to systems wherein the overall transmission bandwidth is a multiple of the basic transmission bandwidth, and wherein the transmission supports fragmented frequency assignment. Such systems are described in Laroia et al “Designing a Mobile Broadband Wireless Access Network”, IEEE Signal Processing Magazine, September 2004.
The prior art frequency hopping schemes as exemplified above are typically not applicable to systems using variable bandwidth single-carrier transmission, since these schemes put additional constraints on the frequency hopping scheme. Such single-carrier transmission is e.g. considered as a candidate scheme in the uplink of Evolved UTRA as described in TR 25.814 “Physical Layer Aspects of Evolved UTRA” (2005-11), part 9.1.1. Both localized and distributed single-carrier schemes are considered. The latter is sometimes also referred to as Interleaved Frequency Division Multiple Access, IFDMA. Similarly, multi-carrier transmission with non-fragmented (localized) or distributed (but regular) spectrum allocation is not supported by the prior art frequency hopping schemes referred to above.
It would be of high value to be able to utilise the advantages offered by frequency hopping, also in systems not using fixed bandwidth allocation.